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Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Derivative
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	$ 54.9		$ 59.7
Interest Rate Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net	0.6
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months	56.9
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	(7.0)	$ 28.8
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge	0.0	0.0
Derivative, Notional Amount	846.3	931.4	744.4	$ 881.4
Designated as Hedging Instrument \| Other Liabilities
Derivative
Gross Derivative Liability	32.3		35.4
Not Designated as Hedging Instrument \| Other Liabilities
Derivative
Gross Derivative Liability	45.5		64.1
Credit Risk Contract \| Not Designated as Hedging Instrument \| Other Liabilities
Derivative
Gross Derivative Liability	0.0		0.0
Credit Risk Contract \| Credit Default Swaps
Derivative
Derivative, Notional Amount	11.9	10.7	11.7	11.4
Credit Risk Contract \| Credit Default Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	11.9		11.7
Forwards
Derivative
Derivative, Notional Amount	11.9	7.6	11.9	8.9
Forwards \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	11.9		11.9
Forwards \| Not Designated as Hedging Instrument \| Other Liabilities
Derivative
Gross Derivative Liability	0.1
Foreign Exchange Contracts \| Not Designated as Hedging Instrument \| Other Liabilities
Derivative
Gross Derivative Liability	22.5		23.8
Foreign Exchange Contracts \| Cash Flow Hedging \| Designated as Hedging Instrument \| Other Liabilities
Derivative
Gross Derivative Liability	10.1		9.4
Foreign Exchange Contracts \| Fair Value Hedging \| Designated as Hedging Instrument \| Other Liabilities
Derivative
Gross Derivative Liability	22.2		26.0
Total Return Swap \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	75.5
Credit Exposure
Derivative
Credit Exposure on Derivatives	0.0		0.7
Receive Fixed/Pay Fixed \| Interest Rate Swaps
Derivative
Derivative, Notional Amount	747.0	663.1	720.8	611.1
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	148.2		148.2
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	210.2		210.2
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	388.6		362.4
Receive Fixed/Pay Variable \| Interest Rate Swaps
Derivative
Derivative, Notional Amount	$ 0.0	$ 250.0	$ 0.0	250.0
Receive Fixed/Pay Variable \| Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount				$ 250.0